问题
I have a List of HashMap's which has key of type Integer and value of type Long.
List<HashMap<Integer, Long>> ListofHash = new ArrayList<HashMap<Integer, Long>>();
for (int i = 0; i < 10; i++) {
HashMap<Integer, Long> mMap = new HashMap<Integer, Long>();
mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L+i));
ListofHash.add(mMap);
}
Now, how do I retrieve the key and value from the list of HashMap?
If using Collection class is the solution, please enlight me.
Update 1:
Actually i am getting the value from the database and putting that into a HashMap
public static List<Map<Integer, Long>> populateMyHashMapFromDB()
{
List<HashMap<Integer, Long>> ListofHash = new ArrayList<HashMap<Integer, Long>>();
for (int i = 0; i < 10; i++) {
HashMap<Integer, Long> mMap = new HashMap<Integer, Long>();
mMap.put(Integer.valueOf(getIntFromDB(i)), Long.valueOf(getLongFromDB(i)));
ListofHash.add(mMap);
}
return ListofHash;
}
where getIntFromDB and getLongFromDB can retreive any int and long values respectively.
Now this is where i want to get my values that i got from DB both keys and values
public void getmyDataBaseValues()
{
List<HashMap<Integer,Long>> ListofHash=populateMyHashMapFromDB();
// This is where i got confused
}
ANSWER This is why Peter Lawrey suggested
public class HashMaps {
public static void main(String[] args) {
// TODO Auto-generated method stub
testPrintHashmapValues(putToHashMap());
testPrintHashmapValuesWithList(putToHashMapWithList());
}
public static Map<Integer, Long> putToHashMap() {
Map<Integer, Long> map = new LinkedHashMap<Integer, Long>();
for (int i = 0; i < 10; i++) {
map.put(Integer.valueOf(i), Long.valueOf(200000000000L + i));
}
return map;
}
public static void testPrintHashmapValues(Map<Integer, Long> map) {
for (Map.Entry<Integer, Long> entry : map.entrySet()) {
System.out.println("key: " + entry.getKey() + " value: "
+ entry.getValue());
}
}
public static List<Map<Integer, Long>> putToHashMapWithList() {
List<Map<Integer, Long>> listOfHash = new ArrayList<Map<Integer, Long>>();
for (int i = 0; i < 10; i++) {
Map<Integer, Long> mMap = new HashMap<Integer, Long>();
mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L + i));
listOfHash.add(mMap);
}
return listOfHash;
}
public static void testPrintHashmapValuesWithList(
List<Map<Integer, Long>> listOfHash) {
for (int i = 0; i < 10; i++) {
Map<Integer, Long> map = listOfHash.get(i);
for (Map.Entry<Integer, Long> entry : map.entrySet()) {
System.out.println(i + " hashMap");
System.out.println("key: " + entry.getKey() + " value: "
+ entry.getValue());
}
}
}
}
My Task is done even without creating a List.
回答1:
Its still not clear to me why you want a list.
public static Map<Integer, Long> populateMyHashMapFromDB() {
Map<Integer, Long> map = new LinkedHashMap<Integer, Long>();
for (int i = 0; i < 10; i++)
map.put(getIntFromDB(i), getLongFromDB(i));
return map;
}
Map<Integer, Long> map = populateMyHashMapFromDB();
Long value = map.get(key);
This collection isn't designed to give you the key/values pairs easily. I fyou need to this functionality I would suggest using a different structure.
Assuming you have a some bad code you cannot change, you can do
List<Map<Integer, Long>> maps = new ArrayList<Map<Integer, Long>>();
Map<Integer, Long> all = new HashMap<Integer, Long>();
for(Map<Integer, Long> map: maps)
all.putAll(map);
for(Map.Entry<Integer, Long> entry: all.entrySet() {
// do something which each key/value.
}
In this example you don't need a List or a Map.
long[] longs = new long[10];
for (int i = 0; i < 10; i++)
longs[i] = i;
int key = 1;
int num = longs[key];
回答2:
You iterate over the list to get the maps, and then iterate over their key set:
public static void main(String[] args) throws NoSuchAlgorithmException {
List<Map<Integer, Long>> ListofHash = new ArrayList<Map<Integer,Long>>();
for (int i = 0; i < 10; i++) {
Map<Integer, Long> mMap = new HashMap<Integer, Long>();
mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L + i));
ListofHash.add(mMap);
}
for (Map<Integer, Long> map : ListofHash) {
for (Integer key : map.keySet()) {
System.out.println(map.get(key));
}
}
}
Note: I've also changed a little your code, using Map instead of HashMap when possible
回答3:
I am assuming you are asking how to get it from ArrayList,
ListofHash.get(0).get(Object key);
Object key means whatever Integer key you want
Good Luck!
回答4:
Something of this effect:
public static Long getValue(Integer key)) {
for (HashMap<Integer, Long> entry : ListofHash) {
if (entry.containsKey(key)) {
return entry.get(key);
}
}
return null;
}
//Retrieve all key/value
for (HashMap<Integer, Long> entry : ListofHash) {
for (Integer key : entry.keySet()) {
System.out.println("Key : " + key + ", value: " + entry.get(key));
}
}
PS Untested.
回答5:
List<HashMap<Integer, Long>> ListofHash = new ArrayList<HashMap<Integer, Long>>();
for (int i = 0; i < 10; i++) {
HashMap<Integer, Long> mMap = new HashMap<Integer, Long>();
mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L+i));
ListofHash.add(mMap);
}
回答6:
You have a total of 10 Maps with only one element stored. And those Maps are stored in a List. You can retrieve those Maps and its elements doing this:
public void testPrintHashmapValues() {
List<HashMap<Integer, Long>> listOfHash = new ArrayList<HashMap<Integer, Long>>();
for (int i = 0; i < 10; i++) {
HashMap<Integer, Long> mMap = new HashMap<Integer, Long>();
mMap.put(Integer.valueOf(i), Long.valueOf(100000000000L+i));
listOfHash.add(mMap);
}
for (int i = 0; i < 10; i++) {
Map<Integer, Long> map = listOfHash.get(i);
for (Map.Entry<Integer, Long> entry : map.entrySet()) {
System.out.println(i + " hashMap");
System.out.println("key: " + entry.getKey() + " value: " + entry.getValue());
}
}
}
Also the variables should not start with uppercase (ListOfHash). Even though java won't complain it is a bad practice and difficult to read code like that because one might think it's a class name instead of a variable.
来源：https://stackoverflow.com/questions/6358853/how-to-get-values-from-list-of-hashmap